1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus in which ultrasonic waves are transmitted into the subject, the ultrasonic waves reflected within the subject are received to obtain received signals, and an image is produced in accordance with the received signals thus obtained.
2. Description of the Related Art
Hitherto, an ultrasonic diagnostic apparatus has been widely used for the purpose of diagnoses of a disease.
FIG. 13 is a diagram used for the explanation of the function of an ultrasonic diagnostic apparatus.
A plurality of-ultrasonic transducers 21 arranged are put onto a surface of the subject 1 (particularly, the human body). Each of the plurality of ultrasonic transducers 21 has an origin 2a. Transmission and reception of ultrasonic beams are performed sequentially along a plurality of scanning lines 2 extending inside of the subject 1 with the origin 2a as a starting point. Here, it is assumed that transmission and reception of ultrasonic beams are performed sequentially along the scanning lines 2 which sequentially shifts in an X-direction shown in the figure. An ultrasonic beam (transmission ultrasonic beam) is formed along the scanning line 2 in such a manner that high voltage pulses each having the associated controlled delay quantity are applied to the whole or a part of plurality of ultrasonic transducers of the plurality of ultrasonic transducers 21, ultrasonic waves of burst waves each having controlled phase owing to a lag in timing of applying of the high voltage pulses are transmitted from the plurality of ultrasonic transducers, and the ultrasonic waves, which are mutually different in their phase, are superimposed on one another. The transmission ultrasonic beam transmitted to the inside of the subject 1 travels inside the subject 1 while being reflected on the respective points of the inside of the subject 1. Ultrasonic waves reflected on the respective points of the inside of the subject 1 are received by the whole or a part of plurality of ultrasonic transducers of the plurality of ultrasonic transducers 21, and are converted into received signals. Those received signals are relatively delayed by the associated controlled delay quantity, respectively and then added together (beamformed), so that a scanning line signal representative of a reception ultrasonic beam extending inside the subject 1 along the scanning line 2 is formed. With respect to the formation of the reception ultrasonic beam, it will be further described later.
It is to be noted that the scanning line 2 implies a central line of the transmission ultrasonic beam or the reception ultrasonic beam thus formed. Consequently, while the scanning line 2 only exists, when the transmission ultrasonic beam or the reception ultrasonic beam, here the scanning line is made up in mind first, and there is provided such an expression that the transmission ultrasonic beam or the reception ultrasonic beam is formed along the scanning line.
In this manner, the formation of the transmission ultrasonic beam and the reception ultrasonic beam is repeatedly performed along the scanning lines 2 sequentially moved, thereby generating an image signal representative of a frame of image. Such an image signal is finally transmitted, for example, to a display device in which an image based on the image signal thus generated is displayed on a display screen of the display device.
Again referring to FIG. 13, the problems of the earlier developed ultrasonic diagnostic apparatus will be described hereinafter.
It is assumed that a cyst 11 exist inside of the subject 1. The cyst implies a portion, such as internal organs (e.g. gall bladder) and a lesion area (e.g. pustule) inside of which water or pus is gathered, in which ultrasonic waves do not almost reflect. In general, ultrasonic waves undergo a strong reflection on a surface 11a of the cyst 11. In the event that a reflection point A, which involves a strong reflection, exists between the cyst 11 and the origin 2a of the ultrasonic transducers 21 on the scanning line 2 extending vertically in the center of the figure, the transmission ultrasonic beam transmitted from the ultrasonic transducers 21 along the central scanning line 2 partially undergoes a reflection on the reflection point A as shown in FIG. 13 with (a), and components passing through the reflection point A without a reflection on the reflection point A further travels inside of the subject 1, and a part of the components is reflected on the surface 11a of the cyst 11. A part of the ultrasonic waves reflected on the surface 11a of the cyst 11 returns, as shown in FIG. 13 with (b), to the ultrasonic transducers 21, but a part of the reflected ultrasonic waves is again reflected on the reflection point A and further reflected on the surface 11a of the cyst 11 once more and then returns, as shown in FIG. 13 with (c), to the ultrasonic transducers 21. In the ultrasonic diagnostic apparatus, a depth position of the reflection position is detected by means of a measurement of the time required since the transmission ultrasonic beam is transmitted until the reflected ultrasonic wave is received by the ultrasonic transducers 21. Consequently, the reflection position of the ultrasonic wave returned to the ultrasonic transducers via the route shown in FIG. 13 with (c) will be recognized as a point A' coming into the cyst 11 by a distance equal to a distance between the reflection point A and the surface of the cyst 11. As a result, on the image, a false echo image appears inside the cyst 11. This phenomenon is called a multiple reflection or multiple echoes. The multiple reflection or multiple echoes is easy to appear in such a site that a strong reflection organization exists at upper portions close to the ultrasonic transducers 21, and areas lower in the reflection factor spread out at lower portions. This phenomenon causes an image quality to be lowered.
Japanese patent laid open gazette Hei.6-125908 discloses, for the purpose of solving the problems of the multiple reflection or multiple echoes, a technical concept that a reception ultrasonic beam, which extends in a slanting direction with respect to a direction in which a transmission ultrasonic beam extends, is formed, and a reflection ultrasonic wave on an area at which the transmission ultrasonic beam and the reception ultrasonic beam intersects each other s received. According to Japanese patent laid open gazette Hei.6-125908, there is provided such an explanation that as if a strong reflection point (corresponding to the reflection point A shown in FIG. 13) exists only in the direction of the transmission ultrasonic beam as shown in FIG. 13. Indeed, however, it happens that a strong reflection point exists also in the direction of the reception ultrasonic beam. Therefore, in such a case, the slanting reception of the ultrasonic beam cannot solve the problems of the multiple reflection or multiple echoes. The slanting reception of the ultrasonic beam will serve to weaken an intensity of the normal reflection ultrasonic wave, thereby lowering S/N of the received signals, in addition to the problems of the multiple reflection or multiple echoes. Also in this respect, the problem as to lowering of the image quality remains.